1. Field of the Invention
This invention relates to electric power systems in which power to a load bus from a utility source is supplemented by, or should the utility source fail, is replaced by, power from an emergency source. More particularly, it relates to a transfer switch for automatically providing power to a load bus from a utility source and/or a microturbine.
2. Background Information
Facilities having a critical requirement for continuous electric power, such as hospitals, certain plant processes, computer installations, and the like, have a standby power source, often a diesel generator. A transfer switch controls connection of the utility lines and the diesel generator to the facility load buses. In many installations, the transfer switch automatically starts the standby generator and connects it to the load bus upon loss of utility power, and reconnects the utility power to the load bus if utility power is reestablished. Some transfer switches effect an open transition between the power sources, that is, one is disconnected from the load bus before the other is connected. Others provide a closed transition wherein the oncoming source is connected to the load bus before the other is disconnected so that both are connected in parallel during the transition. In the latter case, the voltage of the two sources must be in phase and of the same frequency during the closed transition. This can be accomplished passively by waiting for the two voltages to pass through synchronization before initiating the closed transition. Alternatively, the voltage of the standby source can be actively controlled to force it into synchronization with the utility voltage waveform. This active technique can be used to synchronize the two sources for parallel connection to the load bus over extended periods of time.
A relatively new auxiliary power source is the microturbine such as that described in U.S. Pat. No. 5,903,116. Rectified power produced by a gas turbine-driven generator supplies an inverter that produces a three-phase ac output. For stand alone operation, the microturbine operates in a voltage mode. When used to supplement utility power, it is operated in a current mode wherein the voltage is phase locked to the utility voltage.
To my knowledge, there is no automatic transfer switch that accommodates for the unique operating characteristics of a microturbine so that it may be used as an alternative or a supplement to a utility source.